The present invention relates to a developing device for use in an image recorder and of the type including a developing roller and using a single-component developer and, more particularly, to a developing device with an elastic developing roller for which optimal conditions associated with overall electrical characteristics have been determined. Further, the present invention is concerned with a method for producing such a developing roller.
Developing devices applicable to an electrophotographic copier, facsimile apparatus, laser printer or similar image forming apparatus may generally be classified into two types, i.e., a type using a two-component developer which consists of toner and conductive carrier and a type using a single-component developer which lacks carrier, as well known in the art. In any case, the developing device includes a developing roller and develops an electrostatic latent image formed on an image carrier in the form of a photoconductive element by supplying the developer to the latent image via the roller. The single-component type developing device, compared to the two-component type device, is attracting increasing attention because of its slow aging, small-size configuration, and low cost. Especially, various improvements in the developing roller of the single-component type developing device have been reported.
Generally, the developing roller is made up of a metal core, a support layer provided on the metal core, and a dielectric layer provided on the support layer. It has been proposed to arrange on the dielectric layer and in a position associated with the surface portion of the developing roller float electrode portions which are constituted by a number of small electrodes that are insulated from each other, as disclosed in Japanese Patent Laid-Open Publication No. 57-114163 by way of example. With this kind of scheme, a developing electrode effect particular to the carrier of a two-component developer is implemented by the number of small electrodes, i.e., by the developing roller itself to achieve desirable gradation and reproducibility.
Also proposed in relation to a developing device having the above structure is an SNSP (Soft Nonmagnetic Single-Component Development Process) which allows a field effect of the developing roller to effectively act on, among single-component developers, a non-conductive single-component developer. For this purpose, SNSP provides the developing roller with elasticity so that the roller may make contact with the photoconductive element which is rigid. To provide the developing roller with elasticity, the dielectric layer which constitutes the roller in cooperation with the metal core and support layer and, in effect, plays the role of a capacitor may be provided with a substantial thickness. From an electrical standpoint, however, the thickness of the dielectric layer has to be confined to a certain range which insures an electric field for development. It is therefore necessary to determine various conditions associated with the developing roller which would satisfy both of such contradictory requirements. In practice, electrical characteristics of the support layer and dielectric layer, such as resistance and dielectric constant, depend upon the materials of such layers and, yet, they are apt to vary with ambient temperature and humidity. Difficulty has therefore been experienced in selecting various conditions associated with the developing roller, especially optimum conditions for electrical characteristics.
As stated above, with the prior art developing device, it has been almost impracticable to provide the developing roller with elasticity while adequately matching it to the photoconductive element which is rigid, especially to adjust the developing electric field for solid images.